OpenAL Soft's renderer has advanced quite a bit since its start with panned
stereo output. Among these advancements is support for surround sound output,
using psychoacoustic modeling and more accurate plane wave reconstruction. The
concepts in use may not be immediately obvious to people just getting into 3D
audio, or people who only have more indirect experience through the use of 3D
audio APIs, so this document aims to introduce the ideas and purpose of
Ambisonics as used by OpenAL Soft.


What Is It?
===========

Originally developed in the 1970s by Michael Gerzon and a team others,
Ambisonics was created as a means of recording and playing back 3D sound.
Taking advantage of the way sound waves propagate, it is possible to record a
fully 3D soundfield using as few as 4 channels (or even just 3, if you don't
mind dropping down to 2 dimensions like many surround sound systems are). This
representation is called B-Format. It was designed to handle audio independent
of any specific speaker layout, so with a proper decoder the same recording can
be played back on a variety of speaker setups, from quadraphonic and hexagonal
to cubic and other periphonic (with height) layouts.

Although it was developed decades ago, various factors held ambisonics back
from really taking hold in the consumer market. However, given the solid
theories backing it, as well as the potential and practical benefits on offer,
it continued to be a topic of research over the years, with improvements being
made over the original design. One of the improvements made is the use of
Spherical Harmonics to increase the number of channels for greater spatial
definition. Where the original 4-channel design is termed as "First-Order
Ambisonics", or FOA, the increased channel count through the use of Spherical
Harmonics is termed as "Higher-Order Ambisonics", or HOA. The details of higher
order ambisonics are out of the scope of this document, but know that the added
channels are still independent of any speaker layout, and aim to further
improve the spatial detail for playback.

Today, the processing power available on even low-end computers means real-time
Ambisonics processing is possible. Not only can decoders be implemented in
software, but so can encoders, synthesizing a soundfield using multiple panned
sources, thus taking advantage of what ambisonics offers in a virtual audio
environment.


How Does It Help?
=================

Positional sound has come a long way from pan-pot stereo (aka pair-wise).
Although useful at the time, the issues became readily apparent when trying to
extend it for surround sound. Pan-pot doesn't work as well for depth (front-
back) or vertical panning, it has a rather small "sweet spot" (the area the
head needs to be in to perceive the sound in its intended direction), and it
misses key distance-related details of sound waves.

Ambisonics takes a different approach. It uses all available speakers to help
localize a sound, and it also takes into account how the brain localizes low
frequency sounds compared to high frequency ones -- a so-called psychoacoustic
model. It may seem counter-intuitive (if a sound is coming from the front-left,
surely just play it on the front-left speaker?), but to properly model a sound
coming from where a speaker doesn't exist, more needs to be done to construct a
proper sound wave that's perceived to come from the intended direction. Doing
this creates a larger sweet spot, allowing the perceived sound direction to
remain correct over a larger area around the center of the speakers.

In addition, Ambisonics can encode the near-field effect of sounds, effectively
capturing the sound distance. The near-field effect is a subtle low-frequency
boost as a result of wave-front curvature, and properly compensating for this
occurring with the output speakers (as well as emulating it with a synthesized
soundfield) can create an improved sense of distance for sounds that move near
or far.


How Is It Used?
===============

As a 3D audio API, OpenAL is tasked with playing 3D sound as best it can with
the speaker setup the user has. Since the OpenAL API doesn't expose discrete
playback speaker feeds, an implementation has a lot of leeway with how to deal
with the audio before it's played back for the user to hear. Consequently,
OpenAL Soft (or any other OpenAL implementation that wishes to) can render
using Ambisonics and decode the ambisonic mix for a high level of accuracy over
what simple pan-pot could provide.

In addition to surround sound output, Ambisonics also has benefits with stereo
output. 2-channel UHJ is a stereo-compatible format that encodes some surround
sound information using a wide-band 90-degree phase shift filter. This is
generated by taking the ambisonic mix and deriving a front-stereo mix with
with the rear sounds filtered in with it. Although the result is not as good as
3-channel (2D) B-Format, it has the distinct advantage of only using 2 channels
and being compatible with stereo output. This means it will sound just fine
when played as-is through a normal stereo device, or it may optionally be fed
to a properly configured surround sound receiver which can extract the encoded
information and restore some of the original surround sound signal.


What Are Its Limitations?
=========================

As good as Ambisonics is, it's not a magic bullet that can overcome all
problems. One of the bigger issues it has is dealing with irregular speaker
setups, such as 5.1 surround sound. The problem mainly lies in the imbalanced
speaker positioning -- there are three speakers within the front 60-degree area
(meaning only 30-degree gaps in between each of the three speakers), while only
two speakers cover the back 140-degree area, leaving 80-degree gaps on the
sides. It should be noted that this problem is inherent to the speaker layout
itself; there isn't much that can be done to get an optimal surround sound
response, with ambisonics or not. It will do the best it can, but there are
trade-offs between detail and accuracy.

Another issue lies with HRTF. While it's certainly possible to play an
ambisonic mix using HRTF and retain a sense of 3D sound, doing so with a high
degree of spatial detail requires a fair amount of resources, in both memory
and processing time. And even with it, mixing sounds with HRTF directly will
still be better for positional accuracy.
